With the recent progress of a vacuum degassing treatment of molten steel, it has now become easy to make very low-carbon steel through melting, and there has now been an increasing demand for very low-carbon steel strips having a good workability. Among such strips, a very low-carbon steel strip, having Ti and Nb added thereto in combination, which is disclosed for example in Japanese Patent Unexamined Publication No. 59-31827 and Japanese Patent Unexamined Publication No. 59-38337, possesses a very good workability, also has coating-baking hardenability (BH), and is excellent in molten zinc platability, and therefore is now holding an important position.
On the other hand, in order to enhance the strength while maintaining the workability, various attempts have heretofore been made. Particularly, in the case of steel with a tensile strength of 35 to 50 kgf/mm.sup.2 with which the present invention is concerned, P, Si and so on have been added to the steel to increase the strength utilizing a solid solution-strengthening mechanism thereof. For example, Japanese Patent Unexamined Publication No. 59-31827 and Japanese Patent Unexamined Publication No. 59-38337 disclose a method of producing a high-strength cold-rolled steel strip in which Si and P are mainly added to a very low-carbon steel strip having Ti and Nb added thereto, thereby increasing the tensile strength up to the 45 kgf/mm.sup.2 class. Japanese Patent Publication No. 57-57945 discloses a representative prior art technique relating to a method of producing a high-strength cold-rolled steel strip in which P is added to Ti-added, very low-carbon steel. Japanese Patent Unexamined Publication No. 56-139654 discloses a high-strength steel strip based on Nb-added, very low-carbon steel, as well as a method of producing the same.
As described above, P and then Si have heretofore been extensively used as a strengthening element. This is because it has been thought that by adding a small amount of P and Si, the strength can be increased since they have a very high solid solution-strengthening ability, that ductility and deep drawability are not so lowered, and that the cost of the addition is not so increased. Actually, however, when it is intended to achieve the increase of the strength only with these elements, not only the strength but also a yield strength are simultaneously increased markedly, so that a defect in plane shape occurs, and its use for a panel of an automobile is sometimes limited. In the case of applying molten zinc-plating, Si causes a plating defect, and also P and Si greatly lowers the alloying speed, which results in a problem that the productivity is lowered.
On the other hand, it is also known to use Mn and Cr as a solid solution-strengthening element. Japanese Patent Unexamined Publication No. 63-190141 and Japanese Patent Unexamined Publication No. 64-62440 disclose a technique in which Mn is added to a Ti-contained, very low-carbon steel strip, and Japanese Patent Publication No. 59-42742 and the above-mentioned Japanese Patent Publication No. 57-57945 disclose a technique in which Mn and Cr are added to Ti-added, very low-carbon steel; however, (i) the addition of Mn or Cr merely plays an auxiliary role for the main addition elements, P and Si, and therefore the obtained cold-rolled steel strip is high in yield strength as compared with the strength, and besides (ii) they are not added positively for other purposes than the above purpose (i), such as (a) the purpose of enhancing a work hardening rate, (b) the purpose of imparting a BH property, (c) the purpose of enhancing a secondary workability and (d) the purpose of improving the platability of molten zinc-plating.
Further, Japanese Patent Unexamined Publication No. 2-111841 discloses a cold-rolled steel strip and a molten zinc-plated steel strip having a good workability and a baking hardenability in which not less than 1.5% but less than 3.5% Mn is added to very low-carbon steel having Ti added thereto. With the addition of a large amount of Mn, the purpose is to achieve a stable operation of hot rolling due to a lowered Ar.sub.3 transformation point, as well as the uniformity of the metal structure. For the purpose of further enhancing the ductility, the addition of Cr or V of up to 0.2 to 1.0% is also disclosed. However, there is no description from the viewpoint that the addition of a large amount of Mn or Cr improves mechanical properties, and particularly a balance between the strength and the ductility. Furthermore, the amount of addition of Si is determined to be not more than 0.03% in view of a secondary workability, a chemical conversion treatability and a plating adherability. However, Si is an effective solid solution-strengthening element, and in fact it can be added in an amount of more than 0.03% without substantial detriment to such properties.
A steel strip used for a panel of an automobile or the like is strictly required to have a good plane shape in which there occurs neither spring back nor plane strain after the pressing. Incidentally, it is well known that the lower the yield strength is, the better the plane shape is. However, as described in connection with the prior art, generally, the high-strength design of a steel strip involves an extreme increase in yield strength. Therefore, it is necessary to increase the strength while restraining the increase of the yield strength as much as possible.
Furthermore, a steel strip after subjected to press-forming is required to have a dent-preventing property. The dent-preventing property means a resistance of the steel strip to a permanent dent deformation occurring when a stone or the like strikes against an assembled automobile. Where the strip thickness is uniform, the higher a deformation stress after the press forming and the coating baking is, the better the dent-preventing property is. Therefore, in the case where a steel strip have the same yield strength, the higher a work hardenability is in a low strain range, and also the higher the coating-baking hardenability is, the more the dent-preventing property is enhanced.
From the foregoing, a desirable high-strength steel strip used for a panel of an automobile or the like is not so high in yield strength, and is extremely work-hardened, and if possible, has a coating-baking hardenability. Of course, it also need to be excellent in such workability as the average r value (deep drawability) and elongation (bulging property), and further need to be substantially of a non-aging nature at normal temperatures.